9. September 2009 - GPS Chipsets : SiRFAtlas IV Impresses

INTRODUCTIONWhen the SiRFstarIII chipset was released in early 2005, it represented a major breakthrough for GPS as a mainstream consumer product thanks to its high sensitivity engine that made GPS easier to use. Since that time, GPS performance had remained unchanged, with other chipset makers releasing their own high sensitivity chipsets, see the 2007 GPS Chipsets reviewed article, and SiRF adding some "off board" enhancements such as SiRFInstantFix AGPS version 1 and version 2. The announcement of SiRFstarIV in July has brought back some excitement to the field and while we're waiting for it to be released, here is a preview of things to come with the new SiRFAtlasIV PND platform that uses a similar advanced engine.

Line-up - While some will argue that a chipset must be judged on a test bench to remove problems with the design of the device, at the end of the day, what matters is the user experience and real life side by side comparisons of actual devices are the best way to get an idea of what it is. On board for this comparison where no less than 6 GPS systems, starting with 3 dataloggers : AMOD 3080/SS3, Qstarz Q1000P/MTKv1, Qstarz Q1000X/MTKv2 and 3 AIOs/PNDs : Garmin nuvi 1490/STMicro Cartesio, Navigon 8110/SS3 and Takara GP53 with SiRFAtlasIV.

1. ACQUISITIONAs mentioned above, some systems now use AGPS to produce a warm TTFF in the morning in less than ten seconds when they are used regularly (SIF v2 for instance). For the purposes of this comparison, all the systems were left to "settle" for 10 minutes in the open to remove the any impact of this auto-assistance.

2. ACCURACY - RAW TRACKSDriving around the high rise buildings of the budding business complex of Noisy le Grand is a good way to gauge the accuracy of the various systems :

A. Overview

B. Zoom #1

Starting out between high rise buildings, only the SiRFAtlasIV system avoids the classic "multipath" problem (bounced signals that degrade the accuracy) that GPS software algorithms filter out once some speed has been acquired.

C. Zoom #2a

I went around that roundabout twice so the tracks are a bit muddled but do show a problem with the SiRFAtlasIV GPS, let's zoom in...

D. Zoom #2b

...yes something went wrong inside the SiRFstarIV Atlas software. No obvious obstacle in that area, it must have over-compensated for some type of problem, maybe for multipath as we've seen above that it does so aggressively.

E. Zoom #3

Given that this section is under a thick concrete slab, the systems turn in a remarkable performance, with the SiRFAtlasIV producing the smoothest track even it undershoots a bit in the last turn.

F. Zoom #4

More of the same here in a narrow street between and even under tall buildings, with the smoothest track for the SiRFAtlasIV with very good multipath mitigation and the best accuracy.

F. Zoom #5

Again solid performance by all the systems here although the MTKv1 overshoots at the top where the SS3/8110 undershoots and the SiRFAtlasIV overshoots in the bottom.

3. SIGNAL LEVELIn addition to the qualitative approach of analyzing the raw tracks, the quantitative analysis of the signal levels obtained from the NMEA logs can be useful to explain some observations. The raw logger of the Garmin nuvi 1490 and the MTK dataloggers produce a GPX file without signal levels and are therefore not included below.

In line with the generally better track of the SiRFAtlasIV chipset (bar that "bug" in the roundabout) the signal level analysis shows that it beats the SiRFstarIII chipset in all areas, no lost fix, higher number of satellites used in the fix on average, better HDOP (theoretical accuracy).

The last highlighted number, "Average signal level of satellites used in the fix", gives a clue as to how this better performance is being achieved, by lowering the threshold for using satellites and therefore feeding more raw data to the GPS engine. Seems simple enough, but the reason it hadn't been done so far is that weak signals are hard to work with and can quickly generate significant errors in the computed position. This is likely what happened in 2.c. above but overall this new approach has served the SiRFAtlasIV based GPS well!

4. FINAL WORDSAs was sensed in the introduction, exciting times again on the GPS chipset front after 4 years of solid but stagnant performances. SiRF are clearly on to something with their new "navigation software" common to the SiRFAtlasIV, SiRFPrima and SiRFstarIV platforms, smooth and accurate tracks and mitigated multipath, as seen above, and some users have reported problem free performance with cars equipped with coated windshields. Even more surprising is that the SiRFAtlasIV platform uses the SiRFstarIII hardware so the performance should be even better with the new SiRFstarIV hardware. We'll have to keep an eye on potential glitches associated with using very low satellite signals, but based on the overall performance of this first generation SiRFAtlasIV platform, they should be able to manage this.

If you have questions, comments or suggestions, you can use this thread of the forums.